MODELLING THERMAL PERFORMANCE OF BUILDINGS IN COMPOSITE CLIMATE OF INDIA

Abstract

Indoor thermal conditions in buildings can be improved by judicious selection of building components. However, this study is limited to study of thermal performance of parameters i.e. orientation of building, roof type, walls & fenestrations, sunshades, thermal mass and ventilation rate. It is possible to study the influence of these parameters on the indoor environment by evolving a Model. It will facilitate the building designers to study the relative merits of various building designs for the parameters to achieve an optimum building design in the context of thermal comfort. The composite climate of India prevails in northern part of the country, which experiences uncomfortable conditions almost for 06 months during summer. During this period, majority of population depend to harness the natural means of cooling. People use auxiliary means for cooling like air circulators as they cannot afford to spend large amount on air conditioning. In Indian context, thermal performance of the building can be judged by the maximum/minimum indoor air temperature during peak hours (hours in which max./min. temperature occurs outside in summer & winter respectively) and temperature profiles (Outdoor Vs Indoor) for different rooms of a particular building during different seasons of the year. Therefore, in the Indian context, model should indicate the free fluctuations of the internal room temperature for outdoor temperature fluctuations. All basic building parameters, as stated above, have their contribution for the thermal comfort in buildings. Further, this has been revealed from literature survey that research work carried out so far (particularly in India) by the other agencies / individuals in this field is limited only in quantification of thermal performance of a single building element or one design parameter at a time. However, no research work suggests a model to quantify the integrated effect of building components, as stated above, on thermal performance of building for the composite climate. The computer programs like Energy Plus & TRNSYS are being used for energy analysis in developed countries, however use of these tools in India is limited for reasons which have been discussed in this research work subsequently. Therefore, there is a need for research to evolve a model that quantifies the effect of above parameters. This research work intends to evolve a model that quantifies the effect of orientation of building, roof type, walls & fenestrations, sunshades, thermal mass, ventilation rate and outer ambiance on thermal performance of building in an integrated manner. Objectives of this research work are _ • To develop building simulation model by which thermal performance of internal spaces of a building can be assessed in quantitative terms. • To observe thermal performance of a particular building design with variations in the characteristics like materials, thickness of walls, fenestrations & sunshades etc. • To finally recommend the set of optimum variables (favorable parameters) that affect thermal performance of internal spaces. Scope of this Research work would be confined to develop the relevant Model in order to analyze the thermal performance of buildings and to quantify the effect of elements like thermal mass of walls & roof, sunshades/ overhangs; fenestrations on indoor environment in composite climate of INDIA, specially for low-rise residential buildings. The research concentrates on computation and prediction of indoor temperature (DBT) of different rooms of a particular building under normal conditions. Climatic data of Roorkee (2005), as recorded by Meteorological department of National Institute of Hydrology, Roorkee has been used in order to prepare the weather data files for representative day i.e. 21 day of each month. Building, space and other standard as recommended by National Building Code of India 1983 (SP - 7) has been followed. Model has been prepared by considering Forward Approach as described by ASHRAE, which has been formulated on the basis of computational method basically suggested by Indian Code SP: 41 (S & T) - 1987 (Handbook on Functional Requirements of Buildings) for Heat Transmission through Building Sections. Further, in order to prepare mathematical model Heat Balance Theory has also been taken into consideration. Most of the assumptions are supported by ASHRAE or by prEN 15255:2007 (E) Thermal performance of buildings - Sensible room cooling load calculation - General criteria and validation procedures. Mathematical model of the system can be broadly divided in following sub-groups for better understanding _ • Modeling of the SUN - • Azimuth, Altitude & Declination Time Calculations - - Apparent (local) solar time (ASt) - Apparent solar time of sunset (HSs), - • Equation of Time , Apparent solar time of sunrise (HSr) Modeling of Solar Radiation (Heat Influx) - Direct Normal solar radiation (IDn) - Angle of incidence of solar radiation on the surface (i) - Direct solar radiation (lD), Diffuse Solar Radiation (ld), - Sol-air-temperature (TSOi_) Thermal Equilibrium of a Multiple Zone (Room) System involves following computations_ • Development Of System Property Matrices - Development of Heat Capacity [Hc] Matrix - Development of Conductivity Matrix [U] Development of Heat Load Vector {HJ - Inflow From Outer Elements Due To Solar Load - Inflow from Floor - Inflow due to Equipments Load, - Inflow due to Occupants Load - Internal Heat Balance - Inflow due to Air Changes • Modeling of thermal equilibrium (System Heat Balance) • Time marching scheme for hourly calculation of system temperatures A Computer Programme has been prepared incorporating the Model structure in order to produce output data by processing the input data. The program has been prepared in FORTRAN -90 by adopting Modular (structured) Approach. 24 Subroutines and 14 Functions have been formulated in order to accomplish this research. General Output File, Temperature Output File, Solar Calculation File and Shading Calculation File are generated for every simulation study by the programme. To determine the level of precision of results, a reference house of ROORKEE has been selected. Actual temperature readings were recorded on hourly basis in 05 different rooms of that house as well as outdoor temperature was recorded for the same period. The same house with constructional details and specifications was used for indoor temperature simulation from the evolved computer programme. The results of this exercise conclude that the model is capable of predicting Thermal Performance of Buildings within tolerable limits. Model has been developed to have the following characteristic features_ • It is flexible enough to support a wide range of building design, construction types & materials. • Climatic data file of any Indian city (if prepared) can be attached to the model; however, as default settings, the weather data file of Roorkee is attached to the model. • It can predict the thermal performance of different rooms for any hour of the year in the form of Ti (DBT) of that room. • Effect of Fenestrations, sun shading devices and air velocity prevailing outside can be quantified on thermal environment of any room. • Results can be obtained for varying ambient ground reflectivity. • Model accounts for the heat capacity of thermal masses present in rooms. • Occupants' body heat load &Heat generated by the appliances within the rooms is considered on hourly basis. • Variable air changes per hour in any room can be considered for different period of the day or night. • Input files can be prepared in a short time and output files can be obtained in simple format. • Thermal Time lag can be observed on graph (To vs Ti) using output data. • Results can be obtained for the following design parameters / building elements keeping an individual or a set of parameters as variable(s)_ - FENESTRATIONS: Size on different walls &properties of glazing. - GROUND REFLECTIVITY - HEIGHT of BUILDING/ROOMS - HEIGHT of PARAPET over the roof. - ORIENTATION of BUILDING: With respect to Cardinal Points - PROJECTIONS / SUNSHADES: Width & Type on different faces. - ROOF: Material & Treatment. - VENTILATION RATE : Air change per hour - WALL: Material Properties & Thickness. The research culminates with the application of model on three different buildings. Results have been obtained by these exercises for full year; however, results for the month of January & June were analyzed critically. Relevant data & graphs thus produced were analyzed in order to derive inferences form the exercises. As a result of this research work, the Model has been developed in order to determine (in quantitative terms) Thermal Performance of Buildings in Composite Climate of India. Input data preparation method has been made user friendly in easy to use simplified manner. Model provides an opportunity for architects and building designers to formulate their ideas when design is in conceptual stage. With this model, it is possible to study the relative merits of various designs for the same site and to evolve an optimum building design in the context of thermal comfort for composite climate of India.